Add support for ion-tabs

[michaeltintiuc]

Add support for ion-tabs.

The implementation creates only a single Vue component for ion-tabs, the rest of the functionality is kept as a web component.

Custom features:

• Ability to pass a string or an array of strings of route names to match tabs against to ion-tab (via :routes attribute)
• Ability to pass an object containing route data to ion-tab-button, identical to <router-link> (via :to attribute)
• Ability to pass a custom click handler to ion-tab-button

Since ion-tabs require a router, I have exported the existing ionic/vue router.

Quality of Life improvements:

• Fixed broken controller cache, as it would the createApi method without fetching cached objects.
• Updates for @ionic/core v4 stable release
• Updated usage docs for ion-tabs

[grgur]

Fantastic!